The mazzite present in the catalyst of the invention can optionally have undergone post-synthesis modifications such as dealuminization by means of at least one treatment with at least one solution comprising at least one fluorosilicate salt such as ammonium hexafluorosilicate, as described in U.S. Pat. No. 4,503,023, which describes such a treatment for a number of zeolites and, depending on the nature of the zeolite, can produce a degree of dealuminization of at least 30%, or European patent application EP-A-0 573 347, which concerns a mordenite based catalyst thus modified, the concentration of the salt being particularly low with respect to that used in the previous patent, and its use for the isomerization of alkylaromatic compounds containing 8 carbon atoms per molecule.
Surprisingly, a catalyst comprising at least one matrix and at least one zeolite of structure type mazzite, at least partially acid, which has been prepared by at least one treatment with at least one solution of a fluorosilicate of a cation in a proportion of 0.05 to 5 moles per mole of aluminum contained in the dry zeolite, preferably between 0.06 and 4 moles per mole of aluminum contained in the dry zeolite, said prepared zeolite having an Si/Al ratio of the external surface of the zeolite crystals increased of a percentage comprised between 10 and 500%, preferably between 30 and 400% as compared to the Si/Al ratio of the starting zeolite. The catalyst optionally comprising at least one element selected from groups IB and VIII of the periodic classification of the elements, results in improved catalytic performances with respect to prior art catalysts, in particular as regards selectivities, for the dismutation reactions of alkylaromatic hydrocarbons such as toluene, and/or for the transalkylation of alkylaromatic hydrocarbons such as toluene and trimethylbenzenes.
The invention concerns a catalyst comprising at least one matrix (or binder) and at least one zeolite of structure type mazzite, preferably omega zeolite, at least partially, preferably practically completely, in its acid form, comprising silicon and aluminum, which zeolite has been prepared by at least one treatment using at least one solution of a fluorosilicate of a cation in a proportion of 0.05 to 5, preferably 0.06 to 4 moles per mole of aluminium contained in the dry zeolite, said prepared zeolite having an Si/Al ratio of the external surface of the zeolite crystals increased of a percentage comprised between 10 and 500%, preferably between 30 and 400% as compared to the Si/Al ratio of the starting zeolite. The catalyst optionally comprising at least one element selected from the group formed by groups IB and VIII of the periodic classification of the elements.
The matrix is generally selected from members of the group formed by clays (for example from natural clays such as kaolin or bentonite), magnesia, aluminas, silicas, titanium oxide, boron oxide, zirconia, aluminum phosphates, titanium phosphates, zirconium phosphates and silica-aluminas, preferably from members of the group formed by aluminas and clays.
The zeolite with structure type mazzite of the invention is generally selected from the group formed by omega zeolite, mazzite, LZ-202 zeolite or ZSM-4 zeolite, preferably omega zeolite, with a principal pore diameter of about 7.4 .ANG. and with a monodimensional microporous network ("Atlas of Zeolite Structure Types", W. M. Meier and D. H. Olson, 3.sup.rd edition. 1992).
The framework dealuminization step by treatment with at least one solution comprising at least one fluorosilicate salt can optionally take place after a more conventional extraction step such as the dealuminization step described in U.S. Pat. No. 4,780,436.
The catalyst of the invention generally contains 10% to 99%. preferably 20% to 95%, of zeolite with structure type mazzite, preferably omega zeolite, at least partially, preferably practically completely in its acid form. When the catalyst of the present invention contains at least one element selected from the group formed by groups IB and VIII of the periodic classification of the elements, the content of said element(s) is generally in the range 0.01% to 10% by weight, preferably in the range 0.05% to 7% by weight, and more preferably in the range 0.10% to 5% by weight. The complement to 100% generally consists of the matrix in the catalyst.
The invention also concerns the preparation of said zeolite with structure type mazzite and said catalyst.
The zeolite with structure type mazzite comprised in the catalyst used in accordance with the invention, comprising silicon and aluminum, has a global Si/Al atomic ratio in the range 3.2 to 100, preferably in the range 6 to 80, and more preferably in the range 8 to 60, also a sodium content of less than 0.6% by weight with respect to the dry zeolite weight, preferably less than 0.1% by weight.
The zeolite with structure type mazzite of the invention is prepared by dealuminization of an unrefined synthesized zeolite with structure type mazzite using any method which is known to the skilled person, in particular the method described in U.S. Pat. No. 4,780,436 when T is aluminum, i.e., a calcining step is carried out in a stream of dry air, to eliminate the organic structuring agent occluded in the microporosity of the zeolite, followed by at least one ion exchange step using at least one NH.sub.4 NO.sub.3 solution, to eliminate practically all alkaline cations, in particular sodium, present in the cationic position in the zeolite, then at least one framework dealuminization cycle comprising at least one calcining step in the presence of steam at a temperature which is generally in the range 550.degree. C. to 850.degree. C., followed by at least one acid attack step.
The zeolite undergoes at least one treatment with at least one dilute cation fluorosilicate solution, preferably at least one dilute solution of ammonium hexafluorosilicate, generally comprising between 0.05 and 5, preferably between 0.06 and 4, moles per aluminum mole present in the dry zeolite. The external Si/Al ratio is specifically increased, in a percentage of at least 10% and at most 500% and preferably at least 30% and at most 400% as compared to the Si/Al ratio of the starting zeolite.
The framework dealuminization cycle for the zeolite with structure type mazzite, comprising at least one calcining step carried out in steam and at least one attack step in an acid medium, can be repeated as many times as is necessary to obtain the dealuminized zeolite with structure type mazzite with the desired characteristics. Similarly, following calcining in steam, a number of successive acid attack steps using different concentrations of acid solutions can be carried out.
The fluorosilicate used as the dealuminizing agent for the zeolitic framework and as a source of silicon, thus allowing re-insertion of silicon atoms into the crystalline network of the zeolite in place of the aluminum atoms which are extracted, is selected from fluorosilicate salts with the following formula: M.sub.2/x SiF.sub.6 where M is a metallic or non-metallic cation with valency x. The cation M can thus be selected from the group formed by NH.sub.4.sup.+, ammonium alkyls, K.sup.+, Na.sup.+, Li.sup.+, Ba.sup.2+, Mg.sup.2+, Cd.sup.2+, Cu.sup.+, Cu.sup.2+, Ca.sup.2+, Cs.sup.30 , Fe.sup.2+, Co.sup.2+, Pb.sup.2+, Mn.sup.2+, Rb.sup.+, Ag.sup.+, Sr.sup.2+, Zn.sup.2+, Ti.sup.+ and H.sup.+.
Preferably, ammonium hexafluorosilicate is used as it leads to the formation of the ammonium salt (NH.sub.4).sub.3 AlF.sub.6 which is soluble in water and can thus be eliminated easily. In general, the temperature at which the zeolite with structure type mazzite is treated with ammonium hexafluorosilicate is in the range 20.degree. C. to 100.degree. C., preferably in the range 50.degree. C. to 100.degree. C. The quantities of ammonium hexafluorosilicate used are calculated with respect to a zeolite which is dried in a stream of air, at 450.degree. C., for 4 hours. The zeolite with structure type mazzite is treated in the presence of ammonium acetate which buffers the pH of the reaction medium to values which are in the range 4 to 8, preferably in the range 5.5 to 7. at which pH values the Framework of the zeolite is not destroyed by direct acid attack.
After addition of the ammonium hexafluorosilicate solution to the suspension of the zeolite of structure type mazzite in an ammonium acetate solution, the reaction mixture is vigorously stirred at the desired temperature for a period which is in the range 30 minutes to 48 hours, preferably in the range 1 to 2 hours.
The zeolite with structure type mazzite is then filtered at the reaction temperature and washed with abundant quantities of boiling water. The volume of boiling water used to carry out the washes corresponds to v/w=150 ml/g (the ratio v/w is the ratio of the volume of boiling water to the quantity of dry zeolite treated).
After this treatment, the zeolite with modified structure type mazzite undergoes heat treatment to decompose the ammonium cations present in the network and obtain the acid form of the zeolite.
Thus the zeolite with structure type mazzite obtained has a sodium content of less than 2000 ppm by weight with respect to the weight of dry omega zeolite, and a global Si/Al atomic ratio which is in the range 3.2 to 100, preferably in the range 6 to 80, and more preferably in the range 8 to 60.
The external Si/Al ratio of the zeolite thus obtained is in the range of 10 to 500% and preferably 30 to 400% more than the unmodified zeolite.
The characteristics of the zeolite with structure type mazzite can be measured by the following techniques:
the Si/Al atomic ratio is determined by X ray fluorescence and by silicon 29 nuclear magnetic resonance, PA1 the sodium content is determined by atomic absorption; PA1 the elementary cell volume and crystallinity are determined by X ray diffraction, the omega zeolite sample being prepared as described in the method in standard ASTM D3942 80 set up for faujasite.
The catalyst can be prepared using any method which is known to the skilled person. In general, it is obtained by mixing the matrix and the zeolite then forming. The optional element from the group formed by groups IB and VIII of the periodic classification of the elements can be introduced either before forming, or during mixing, or to the zeolite itself before mixing it, or, as is preferable, after forming. Forming is generally followed by calcining, generally at a temperature which is in the range 250.degree. C. to 600.degree. C. The optional element from the group formed by groups IB and VIII of the periodic classification of the elements can be introduced after said calcining step. In all cases, the element is generally chosen to be deposited either, as is preferable, practically completely on the zeolite, or practically completely on the matrix, or partially on the zeolite and partially on the matrix, the choice being effected, in a manner which is known to the skilled person, by means of the parameters used during said deposition, such as the nature of the precursor selected to effect said deposition.
The element from groups IB or VIII, preferably selected from the group formed by Ag, Ni and Pt, and more preferably Ni, can also be deposited on the zeolite-matrix mixture which has been pre-formed using any procedure which is known to the skilled person. Such deposition is generally carried out by the techniques of dry impregnation, ion exchange(s) or co-precipitation. When ion exchange is carried out using precursors based on silver, nickel or platinum, the salts which are generally used are silver salts such as chlorides or nitrates, a tetramine complex of platinum, or nickel salts such as chlorides, nitrates, acetates or formates. The ion exchange technique can also be used to deposit the metal directly on the zeolite powder before optional mixing with a matrix.
When the catalyst contains a plurality of metals, these latter can be introduced either in the same way or using different techniques, before or after forming and in any order. When the technique used is ion exchange, a plurality of successive exchanges may he necessary to introduce the required quantities of metals.
As an example, one preferred method for preparing the catalyst of the invention consists of mixing the zeolite in a wet matrix gel (generally obtained by mixing at least one acid and a matrix powder), for example alumina, for the time required to obtain good homogeneity of the paste thus produced, i.e., for about ten minutes, for example, then passing the paste through a die to form extrudates with a diameter which is, for example, in the range 0.4 to 4 mm. After oven drying for several minutes at 100.degree. C. and after calcining, for example for 2 hours at 400.degree. C. the optional element, for example nickel, can be deposited, for example by ion exchange, said deposit being followed by final calcining, for example for 2 hours at 400.degree. C.
The catalyst of the invention is generally formed so that the catalyst is preferably in the form of pellets, aggregates, extrudates or spherules, depending on its use.
Catalyst preparation is generally finished by calcining, termed final calcining, normally at a temperature which is in the range 250.degree. C. to 600.degree. C. preferably preceded by drying, for example oven drying, at a temperature which is generally in the range from ambient temperature to 250.degree. C. preferably in the range 40.degree. C. to 200.degree. C. The drying step is preferably carried out during the period of temperature rise required to carry out the calcining step.
The invention also concerns the use of the catalyst for the dismutation of alkylaromatic hydrocarbons, preferably for the dismutation of toluene to produce benzene and xylenes, and/or for the transalkylation of alkylaromatic hydrocarbons. preferably transalkylation of generally C.sub.9.sup.+ alkylaromatic hydrocarbons (i.e., containing at least 9 carbon atoms per molecule), such as transalkylation and/or dismutation of toluene and/or C), alkylaromatics to produce xylenes. The feed for such a process can comprise 0 to 100% of C.sub.9.sup.+ alkylaromatics and 0 to 100% of toluene.
The operating conditions are generally as follows: a temperature which is in the range 250.degree. C. to 600.degree. C., preferably in the range 330.degree. C. to 500.degree. C.: a pressure which is in the range 10 to 60 bar, preferably in the range 20 to 45 bar; a supply space velocity, expressed in kilograms of feed introduced per kilogram of catalyst per hour, in the range 0.1 to 10, preferably in the range 0.5 to 4: and a hydrogen to hydrocarbons molar ratio which is in the range 2 to 20, preferably in the range 3 to 12.
The following examples illustrate the invention without in any way limiting its scope.